1. Field of the Invention
This invention relates to measuring the amount of a bulk material contained in a storage tank or the like.
2. Description of the Prior Art
There is a need to quickly and accurately measure the amount of a bulk material contained in a storage tank. Of particular concern are granular or particulate types of bulk materials, such as dry cement, sand, barite (drilling mud), flour, grain, etc.
Materials such as dry cement and barite are commonly used in bulk in the oil well drilling industry. It is often necessary to measure the weight or volume of such bulk materials on a floating rig, such as an offshore drilling ship or platform. For example, in offshore drilling there is a need to measure whether a storage tank on a platform contains enough cement before a new job is started, in order to avoid running out of material in the middle of the job.
In one method currently used for measuring the contents of storage tanks, a worker manually measures the remaining height of the material by climbing a ladder to the top of the tank, opening the tank, and dropping in a calibrated rope with a paddle or "stop" on the end. The length of the rope indicates the level of the material remaining in the tank, and this measurement can be used to figure the height, volume or weight of material remaining in the tank. Objections to this method are the inconvenience, lack of safety, and time-consuming nature of the task.
In another technique currently used for measuring the contents of a storage tank, the entire tank is mounted on "load cells" in the form of strain gauges on the feet of the tank. The tank is measured both empty and with a load, and the weight of the contents is the difference between the two weight measurements. One objection to this method is that the tank must be completely supported by the "load cell" system. No stiffness can be permitted in the connected plumbing, or other attachments, or a weighing error will result. Instead, flexible joints must be used, and the total cost of implementing such a system is several thousands of dollars. Use of the tank supported by such "load cells" may be suitable for fixed installations, but it is not suitable for use on movable installations, such as offshore drilling ships, semisubmersibles, or other non-static platforms. The accelerations and changes in center of gravity from wave action and the like cause errors for which compensation is difficult.